Researchers report milestone for antiferromagnetic spintronics

Researchers from the University of Tokyo and CREST (Japan Science and Technology Agency) have explored the world of spintronics and other related areas of solid state physics with a focus on antiferromagnets. The team has reported, in its recent study, the experimental realization of perpendicular and full spin–orbit torque switching of an antiferromagnetic binary state.

The team used the chiral antiferromagnet Mn3Sn, which exhibits the magnetization-free anomalous Hall effect owing to a ferroic order of a cluster magnetic octupole hosted in its chiral antiferromagnetic state. They fabricated heavy-metal/Mn3Sn heterostructures by molecular beam epitaxy and introduce perpendicular magnetic anisotropy of the octupole using an epitaxial in-plane tensile strain. By using the anomalous Hall effect as the readout, the team demonstrated 100% switching of the perpendicular octupole polarization in a 30-nanometre-thick Mn3Sn film with a small critical current density of less than 15 megaamperes per square centimeter. Their theory is that the perpendicular geometry between the polarization directions of current-induced spin accumulation and of the octupole persistently maximizes the spin–orbit torque efficiency during the deterministic bidirectional switching process. The team's recent work provides a significant basis for antiferromagnetic spintronics.

"Like ferromagnets, antiferromagnets' magnetic properties arise from the collective behavior of their component particles, in particular the spins of their electrons, something analogous to angular momentum," said University of Tokyo's Professor Satoru Nakatsuji. "Both materials can be used to encode information by changing localized groups of constituent particles. However, antiferromagnets have a distinct advantage in the high speed at which these changes to the information-storing spin states can be made, at the cost of increased complexity."

"Some spintronic memory devices already exist. MRAM (magnetoresistive random access memory) has been commercialized and can replace electronic memory in some situations, but it is based on ferromagnetic switching," said Project Associate Professor Tomoya Higo. "After considerable trial and error, I believe we are the first to report the successful switching of spin states in antiferromagnetic material Mn3Sn by using the same method as that used for ferromagnets in the MRAM, meaning we have coaxed the antiferromagnetic substance into acting as a simple memory device."

This method of switching is called spin-orbit torque (SOT) switching and it's cause for excitement in the technology sector. It uses a fraction of the power to change the state of a bit (1 or 0) in memory, and although the researchers' experiments involved switching their Mn3Sn sample in as little as a few milliseconds (thousandth of a second), they are confident that SOT switching could occur on the picosecond (trillionth of a second) scale, which would be orders of magnitude faster than the switching speed of current state-of-the-art electronic computer chips.

"We achieved this due to the unique material Mn3Sn," said Nakatsuji. "It proved far easier to work with in this way that other antiferromagnetic materials may have been."

"There is no rule book on how to fabricate this material. We aim to create a pure, flat crystal lattice of Mn3Sn from manganese and tin using a process called molecular beam epitaxy," said Higo. "There are many parameters to this process that have to be fine-tuned, and we are still refining the process to see how it might be scaled up if it's to become an industrial method one day."

Posted: Jul 21,2022 by Roni Peleg